Field of the Invention
Solid state devices and integrated circuits are now routinely fabricated with sub-micron or even nanometer scale components. These reductions in scale have led to great improvements in the operating characteristics of solid state devices. However, the small size of the device components has also led to new problems in device fabrication.
Traditionally, liquid etching techniques were used in device manufacture. However, since these techniques are limited to the fabrication of components with lateral dimensions of a micron or greater, they cannot be used for the production of nano-scale devices. Instead, dry etching techniques are now extensively used in the manufacture of solid state devices.
Of these dry etching techniques, plasma enhanced etching ("plasma etching") is very well suited to the fabrication of metal and semiconductor components on the nanometer scale. For this reason, plasma etching has become a commercially valuable technique. Any improvement in the productivity of this process would be a significant breakthrough.
A conventional plasma etching reactor includes a reactor vessel and a means for producing a plasma within the reactor vessel. The plasma may be produced either inductively, e.g., using an inductive RF coil, or capacitively, e.g., using a parallel plate glow discharge reactor.
The conventional steps involved in plasma etching are as follows. A mask is overlaid on an exposed surface of a wafer to be etched, and the wafer, or a batch of wafers, is then placed in the reactor vessel. Etching gases are then introduced into the reactor vessel and a plasma is ignited. During processing, the reactive species in the plasma etch the exposed surface of the metal or dielectric material.
At the molecular level, the etch process is a reaction between the reactive species in the plasma and exposed surface layers of the wafer. This reaction yields etch byproducts: small volatile molecules that desorb from the substrate surface and subsequently diffuse away into the reactor vessel. Most of these volatile byproducts are then pumped out of the reactor vessel. Unfortunately, these volatile byproducts can react with water vapor, oxygen, and organic contaminants that may be present in the reactor vessel and thereby form heavier, less volatile byproducts. In addition, some non-volatile species generated as a result of ion bombardment or sputtering can deposit on the vessel walls.
These heavier byproducts, together with excess coating polymer and various contaminants, can deposit on the inner surfaces of the reactor vessel. Each time a batch of wafers is processed, a fresh layer of byproducts and contaminants deposits on the inner surfaces of the reactor vessel. Eventually, these deposits become thick enough to flake off and detach from the reactor walls. This is a major source of particulate contaminants. These contaminants are very harmful to the fabrication process because they lodge in the mask or on the wafer and produce defects. As the size of the etched features becomes smaller, the effects of the particulate contaminants becomes more pronounced, and the need to eliminate these contaminants becomes more important.
Currently, two cleaning methods are used to remove the deposit buildup on the inner surfaces of the reactor walls: the wet cleaning method and the dry cleaning method.
In the wet cleaning method, the reactor is taken off line, dismantled, and the deposit buildup is physically or chemically removed. This cleaning method has at least two major drawbacks. First, the cleaning may take up to 24 hours in which time no processing can take place. Second, the dismantling, mechanical or chemical cleaning, and reassembling of the reactor are labor-intensive and complicated procedures, and can themselves lead to additional sources of contamination.
In the dry cleaning method, the processing of the wafers is alternated with a "dry clean" run. In the dry clean run, a dummy wafer is placed in the reactor vessel, the reactor is then charged with a mixture of cleaning gasses, and a plasma is ignited. The deposits on the inner surface of the reactor vessel are then chemically removed by reaction with reactive species and ion bombardment in the cleaning gas plasma. The dry clean method also has several major drawbacks. First, to maintain a high level of cleanliness in the reactor, it is preferable to carry out a cleaning run after each batch of wafers is processed. This results in considerable down time in which no productive processing can take place. Second, the gasses used in the cleaning run can be corrosive to some of the reactor vessel components. Third, dry clean species remaining in the chamber after a dry clean run can adversely impact subsequent wafer processing.
For these reasons, there is a real need for a reactor cleaning method that neither interrupts plasma processing nor involves potentially damaging mechanical or chemical processes. The present invention fills both of these needs and greatly improves the productivity of plasma processing reactors.